Isocyanates are widely used as production raw materials of such products as polyurethane foam, paints and adhesives. The main industrial production process of isocyanates involves reacting amines with phosgene (phosgene method), and nearly the entire amount of isocyanates produced throughout the world are produced according to the phosgene method. However, the phosgene method has numerous problems.
Firstly, this method requires the use of a large amount of phosgene as raw material. Phosgene is extremely toxic and requires special handling precautions to prevent exposure of handlers thereof, and also requires special apparatuses to detoxify waste.
Secondly, since highly corrosive hydrogen chloride is produced in large amounts as a by-product of the phosgene method, in addition to requiring a process for detoxifying the hydrogen chloride, in many cases hydrolytic chlorine is contained in the isocyanates produced, which may have a detrimental effect on the weather resistance and heat resistance of polyurethane products in the case of using isocyanates produced using the phosgene method.
On the basis of this background, a process for producing isocyanates has been sought that does not use phosgene. One example of a method for producing isocyanate compounds without using phosgene that has been proposed involves thermal decomposition of carbamic acid esters. Isocyanates and hydroxy compounds have long been known to be obtained by thermal decomposition of carbamic acid esters (see, for example, Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870). The basic reaction is illustrated by the following formula:R(NHCOOR′)a→R(NCO)a+aR′OH  (1)(wherein R represents an organic residue having a valence of a, R′ represents a monovalent organic residue, and a represents an integer of 1 or more).
The thermal decomposition reaction represented by the above-mentioned formula is reversible, and in contrast to the equilibrium thereof being towards the carbamic acid ester on the left side at low temperatures, the isocyanate and hydroxy compound side becomes predominant at high temperatures. Thus, it is necessary to carry out the carbamic acid ester thermal decomposition reaction at high temperatures. In addition, in the case of alkyl carbamates in particular, since the reaction rate is faster for the reverse reaction of thermal decomposition, namely the reaction by which alkyl carbamate is formed from isocyanate and alcohol, the carbamic acid ester ends up being formed before the isocyanate and alcohol formed by thermal decomposition are separated, thereby frequently leading to an apparent difficulty in the progression of the thermal decomposition reaction.
On the other hand, thermal decomposition of alkyl carbamates is susceptible to the simultaneous occurrence of various irreversible side reactions such as thermal denaturation reactions undesirable for alkyl carbamates or condensation of isocyanates formed by the thermal decomposition. Examples of these side reactions include a reaction in which urea bonds are formed as represented by the following formula (2), a reaction in which carbodiimides are formed as represented by the following formula (3), and a reaction in which isocyanurates are formed as represented by the following formula (4) (see, Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870 and Journal of American Chemical Society, Vol. 81, p. 2138, 1959):

In addition to these side reactions leading to a decrease in yield and selectivity of the target isocyanate, in the production of polyisocyanates in particular, these reactions may make long-term operation difficult as a result of, for example, causing the precipitation of polymeric solids that clog the reaction vessel.
Various methods have been proposed to solve such problems. For example, a method for producing polyisocyanate has been proposed in which an alkyl polycarbamate, in which ester groups are composed of alkoxy groups corresponding to a primary alcohol, is subjected to a transesterification reaction with a secondary alcohol to produce an alkyl polycarbamate in which the ester groups are composed of alkoxy groups corresponding to the secondary alcohol, followed by thermal decomposition of the alkyl polycarbamate (see, for example, International Publication No. WO 95/23484). It is described in this method that the thermal decomposition temperature of the alkyl polycarbamate can be set to a lower temperature by going through an alkyl polycarbamate in which the ester groups are composed of alkoxy groups corresponding to the secondary alcohol, thereby resulting in the effect of being able to inhibit precipitation of polymeric solid. However, the reverse reaction rate between the polyisocyanate formed by the thermal decomposition reaction of the alkyl polycarbamate and the secondary alcohol is still fast, thereby leaving the problem of inhibiting the formation of alkyl polycarbamate by the reverse reaction unsolved.
An alternative method has been disclosed whereby, in the production of aromatic isocyanates, for example, an aromatic alkyl polycarbamate and an aromatic hydroxy compound are subjected to a transesterification reaction to produce an aromatic aryl polycarbamate followed by thermal decomposition of the aromatic aryl polycarbamate to product an aromatic isocyanate (see, for example, U.S. Pat. No. 3,992,430). This method describes the effect of being able to set the thermal decomposition temperature to a lower temperature by going through an aromatic aryl polycarbamate. However, in the case of this aromatic aryl polycarbamate as well, under temperatures like those at which the transesterification reaction or thermal decomposition reaction is carried out, there are many cases in which side reactions like those described above still occur, there leaving the problem of improving isocyanate yield unsolved. Moreover, thermal decomposition of N-substituted aromatic urethanes in the gaseous phase or liquid phase is known to frequently result in the occurrence of various undesirable side reactions (see, for example, U.S. Pat. No. 4,613,466).